TEIID LIZARDS OF THE GENUS CXEMIDOrHORUS 5 



conditions. Furthermore, it has been forced to adjust itself to en- 

 vironmental modifications brouiiht about by the extension of its 

 raniie throuirh its own miirrations or by the ao:oncy of frcoloii-ical 

 change. Throughout the area of its distribution the individuals 

 concerned must have reached a satisfactory physiological equilibrium 

 with the environment — at least in regard to such prime factors as 

 temperature, moisture, food, shelter, and breeding sites. Failure to 

 do this would have meant restriction of the range, foUoAved b}^ a 

 steady decrease in the number of individuals, and ultimately it would 

 have spelled extinction. Environmental and mutational differences 

 are usually slight in geographically adjacent populations wdiich show 

 a continuous distribution. The appearance of these differences gives 

 incejition to "local races."' Aithough such units are generally not 

 worthy of even subspecific recognition, it appears that they may 

 possess a high degree of evolutionar}' significance. TIuis, it seems 

 logical to believe that the complete isolation of one of these local 

 phases is often the most important step in that benign process which 

 culminates in the birth of a new taxonomic entity. In fact it ap- 

 pears from this study that at least the greatest percentage of new 

 taxonomic forms do arise only after continued isolation from the 

 parent stock, during which time even slight mutations may have be- 

 come of real significance through inbreeding and accretion. 



Because of the fact that island forms are more homogenous it seems 

 permissible to diagnose certain new insular species, or to retain those 

 already described, on a smaller sum total of differences than that 

 necessary to discriminate between mainland populations of a similar 

 stock. As previous!}^ mentioned, the one criterion to be used in mak- 

 ing a separation is constancy. 



When the discrimination is betAveen a mainland form and a recently 

 isolated island type, however, it is often extremely difficult to find 

 reliable characters upon Avhich to base a separation. Here the 

 island population may, even after a very careful comparison with 

 i-epresentatives from the neighboring mainland, present a seemingly 

 distinctive set of characters. This may cause one not thoroughly 

 familiar Avith the actual range of Aariation of the mainland stock to 

 describe a ncAv insular species (or subspecies) Avhose representatives 

 uuiy proA'e to be nothing but duplicates of those of the original form 

 which dAvell in more or less distant regions wdiere there are, perhaps, 

 similar bionomic conditions. 



In the present instance specimens of the commoner forms have been 

 ])ersonally examined at all of the leading American museums both 

 east and Avest and specimens of the rarer forms and the more critical 

 material haA'e been freely loaned for examination at a central point. 

 This method has given the advantage of comparison of pertinent 

 specimens without the disadvantage on the part of museums at least. 



